Sun’s loops tell a tangled tale

THE towering loops of plasma in the sun’s corona – some of them hundreds of thousands of kilometres high – are supposed to follow the sun’s magnetic field. But comparing individual loops with models of the underlying magnetic field lines has revealed that the two don’t match up.

James Klimchuk of the Naval Research Laboratory in Washington DC and his colleagues analysed images of coronal loops taken by NASA’s TRACE spacecraft. They found that the loops were uniformly thick and symmetrical at both anchor points on the sun’s surface. Then they calculated the thickness of the corresponding magnetic flux tubes, or bundles of field lines, using data from the SOHO spacecraft. Surprisingly, the calculations showed that many of the tubes were considerably thicker at one anchor point than at the other, and also expanded twice as much with height as the corresponding coronal loops.

Klimchuk thinks the discrepancy arises because existing models of the sun’s magnetic field are too simple. “Within the sun the convection currents stir up the plasma, which carries the magnetic [flux tubes] along for the ride,” he says. This causes the tubes to tangle up, and since models don’t account for this, they don’t match up with the coronal loops.

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